1. Field of the Invention
The invention relates to digital communications. More particularly, the invention relates to noise estimation in a communication receiver.
2. Description of the Related Art
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems. For example, a CDMA system provides increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards.
A communication system 100 is shown in FIG. 1 as a CDMA wireless phone system. The communication system 100 has one or more base stations, 110a and 110b, here shown as antenna systems typical of a wireless phone system. Although only two base stations 110a, 110b are shown, it is understood that the communication system 100 may support any number of base stations. Each base station 110a, 110b provides coverage for a corresponding cell 120a, 120b. The coverage areas or cells 120a, 120b supported by the two base stations 110a, 110b are shown to be overlapping. However, it is understood that where more than one base station is supported in the communication system 100, the cells supported by each base station may or may not overlap. Additionally, the cells of any three or more base stations may have some common coverage areas or may be mutually exclusive.
Since the operation of the communication system 100 within each cell is substantially identical, the discussion will focus on the operation within a single cell. A base station 110a supports coverage over a corresponding cell 120a. There may be one or more Mobile Stations (MS) 130a, 130b, within the cell 120a simultaneously communicating with the base station 110a. The MS 130a, 130b are shown as portable phones but it is understood that the MS 130a, 130b may be portable phones, mobile phones operating within vehicles, fixed position phones, wireless local loop phones, or any other type of communication device. The base station 110a communicates to each MS 130a, 130b, over a forward link channel and each MS 130a, 130b communicates to the base station 110a over a reverse link channel. The communication links may be over a continuously active channel or may allow for DTX. The base station 110a also communicates with a Base Station Controller (BSC) 150 that provides the communication link to a Public Switched Telephone Network (PSTN) not shown.
The CDMA standards provide specific details concerning the generation of the various channels supported on each of the forward and reverse links. Typical generation of a forward traffic channel and a forward pilot channel are shown, respectively, in the functional block diagrams of FIG. 2 and FIG. 3.
Forward traffic channel generation is shown in the functional block diagram of FIG. 2. The figure shows a generalized functional depiction of Forward Traffic channel generation. Forward channels may not all be generated identically with the block diagram shown in FIG. 2. Data to be transmitted on the forward traffic channel is coupled to a convolutional encoder 204. The convolutional encoder 204 is used to provide Forward Error Correction (FEC). The constraint length and rate of the convolutional encoder 204 may vary according to the particular standard and configuration of the communication system. Alternatively, a turbo encoder may substitute for the convolutional encoder 204 in systems that provide for turbo encoding.
The output of the encoder 204, whether convolutional or turbo, is coupled to an interleaver 208. The forward link signal is interleaved at the BS in order to lessen the effects of a burst of errors that may be caused, for example, by a fast signal fade due to destructively combining multipaths at a receiver front end. Interleaving the symbols before transmission and deinterleaving after reception causes bursts of errors to be spread out in time and to appear to the decoder as if they were random errors. Interleaving is typically performed on a frame basis when block interleaving is performed. The output of the interleaver 208 is coupled to a first input of a first modulo two adder 210.
A long PN code generator 230 generates a pseudo random noise sequence based on a pseudo noise sequence masked in part by an electronic serial number of the specific MS for which the data is intended. The unique output from the long PN code generator 230 is coupled to a first decimator 232 used to reduce the rate of the signal from the long PN code generator 230 to coincide with the symbol rate output from the interleaver 208. The output of the first decimator 232 is coupled to a second input of the first modulo two adder 210. The output of the first modulo two adder 210 represents the interleaved data scrambled by the PN sequence. The scrambled output of the first modulo two adder 210 is then coupled to a first input of a multiplexer (MUX) 220.
A single bit output from a Power Control section 224 is provided as a second input to the MUX 220. The power control bit is punctured onto the scrambled data symbols and is used within a closed power control to instruct the MS to increase or decrease its transmit power. The location of the punctured bit is pseudo randomly determined using the long PN sequence.
The pseudo random sequence output from the first decimator 232 is coupled to an input of a second decimator 234. The second decimator 234 further reduces the rate of the PN sequence to a desired power control puncture rate. The PN sequence output from the second decimator 234 is coupled to a control input of the MUX 220. Thus, the PN sequence provided at the control input to the MUX 220 directs the MUX 220 to puncture the power control bit into the scrambled data symbols in a pseudo random location.
The output of the MUX 220 is coupled to an input of a second modulo two adder 240 used in the direct spreading of the data symbols. A Walsh code generator 242 provides a single Walsh code sequence, that is assigned to the particular Traffic Channel, to a second input of the second modulo two adder 240. The second modulo two adder 240 directly spreads each input symbol with the provided Walsh code. If the Walsh code length is sixty-four, the second modulo two adder 240 outputs a sequence of sixty four chips for each input symbol. The output from the second modulo two adder 240 is the modulo two sum of the Walsh code with the symbol and thus, sixty four chips are outputted in the time span of a single symbol.
The spread output from the second modulo two adder 240 is provided to two parallel paths that are used to generate the in phase (I) and quadrature (Q) signal components. Alternatively, as in some radio configurations of CDMA 2000, the symbols may be Quadrature Phase Shift Keyed and separate in phase and quadrature symbols may be provided to corresponding I and Q signal paths. The output of the second modulo two adder 240 is coupled to an in phase adder 252. The name of the adder denotes the function of adding the signals in the in phase path and does not denote functionality with respect to any specific phase of the signal. An Offset I PN generator 262 is coupled to a second input of the in phase adder 252. The output of the in phase adder 252 represents the in phase data signal and is coupled to a filter (not shown) and a modulator (not shown) for generation of the in phase (I) modulated signal.
Similarly, the output of the second modulo two adder 240 is coupled to an input of a quadrature adder 254. An Offset Q PN generator 264 is coupled to a second input of the quadrature adder 254. The output of the quadrature adder 254 represents the quadrature data signal and is coupled to a filter (not shown) and quadrature modulator (not shown) for generation of the quadrature (Q) modulated signal.
The Offset I and Q PN generators, 262 and 264, are short PN sequences used to isolate one cell or sector from another. The offset enables reuse of the Walsh codes in every sector.
The Pilot Channel can be viewed as a special case of Traffic Channel generation. The Pilot Channel is used to provide a receiver with time, phase, and signal strength information. It is not intended to carry user data. Typical Pilot Channel generation is shown in FIG. 3.
Referring to FIG. 3, all zeros are provided as the input to the Pilot Channel Generator 300. The Pilot Channel carries no information on it. The all zeros input is coupled to a first input of a modulo two adder 310. A Walsh code generator 320 is coupled to a second input of the modulo two adder 310. The modulo two adder 310 normally operates to spread the input signal by the Walsh code. However, the Walsh code used for the Pilot Channel is the zero Walsh code. Thus, the output of the modulo two adder 310 still represents all zeros. The symbols used in the Pilot Channel are Bi-Phase Shift Keyed unlike the alternatives available for forward traffic generation. Only an in phase (I) signal component is represented by all zeros. There is no Q signal component. Thus, the same symbols are routed to both the I and Q signal paths for generation of the QPSK chips. This signal is coupled to a first input of an in phase adder 352. An Offset I PN generator 362 is coupled to a second input on the in phase adder 352. The output of the in phase adder 352 represents the I Pilot data that, when modulated, represents the I Pilot signal.
The output of the modulo two adder 310 is also coupled to a first input of a quadrature adder 354. An Offset Q PN generator 364 is coupled to a second input of the quadrature adder 354. The output of the quadrature adder 354 represents the Q Pilot data that, when modulated, represents the Q Pilot signal.
The Offset I and Q generators, 352 and 354, are the same short PN sequences used in the forward traffic generation and are used to isolate one sector from another. A receiver may recover the short PN offset used in Traffic Channel generation by recovering the short PN offset from the Pilot Channel.
The differences in Traffic Channel and Pilot Channel generation allow a receiver to implement different configurations for extracting the desired information from the respective channels. In addition to extracting information contained in each channel, the receiver may also generate signal metrics in order to assist in closed loop power control. A typical metric used by a receiver in closed loop power control is energy per bit to noise power ratio (Eb/Nt). The receiver measures or estimates an energy per bit value as well as a noise power value in order to calculate the ratio. A signal energy value may be calculated as part of the signal demodulation processing. However, the implementation of a noise power estimator may take various configurations. When the receiver is implemented as a rake receiver having a plurality of fingers, demodulated signals are coherently combined from each of the fingers. A noise power estimate needs to take into account the statistics of the processing of the received signals. Ideally, the statistics of the noise estimate processing track the statistics of the received signal processing. When the statistics of the noise estimate processing do not track those of the signal processing erroneous signal metrics may result. What is needed is a noise power estimation implementation that provides an accurate measurement of the received noise power over all receiver operating conditions.